There are presently available on the market place various devices and apparatuses for electrically testing an individual light bulb after it has been physically removed from its socket. Apparatus is also available on the market for testing series-connected Christmas tree light bulbs, and the like, by physically placing an alternating current line voltage sensor in close proximity to the particular light bulb desired to be tested. However, such a device is merely an electromagnetic field strength detection device which may remain in an "on" condition whenever the particular bulb desired to be tested is physically located in close proximity to another light bulb or bulbs on the Christmas tree.
In fact, light bulb manufacturers have also attempted to solve the problem of bad bulb detection by designing each light bulb in the string in a manner whereby the filament in each light bulb is shorted by various mechanisms and means whenever it burns out for any reason, thereby preventing an open circuit condition to be present in the socket of the burned-out bulb. However, in actual practice, it has been found that such short circuiting feature within the bulb does not always operate in the manner intended, resulting in the entire string going out whenever but a single bulb burns out.
In U.S. Pat. No. 5,539,317, entitled CIRCUIT TESTER FOR CHRISTMAS TREE LIGHT SETS and filed on Nov. 7, 1994 by the same applicant as the instant application, there is disclosed therein a novel, hand held and battery operated device which is capable of testing each light bulb in a string without the necessity of removing the bulb from its socket, thereby readily locating the burned out bulb which caused the entire string of bulbs to go out.
Even though each of the foregoing techniques have met with some limited success, none of such devices and techniques have yet been able to further solve the additional problems of the entire string of lights going out as a direct result of either a defective socket, a light bulb being improperly placed in the socket, a broken or bent wire of a light bulb, or whenever a light bulb is either intentionally removed from its socket or is merely dislodged from its socket during handling or from movement after being strung on the Christmas tree, particularly in outdoor installations subject to wind or other climatic conditions.
U.S. Pat. No. 4,450,382 utilizes a single Zener or "avalanche" type diode which is electrically connected across each series-connected direct-current ("D.C.") lamp bulb used by military vehicles, strictly for so-called "burn-out" protection for the remaining bulbs whenever one or more bulbs burns out for some reason. It is stated therein that the use of either a single or a plurality of parallel and like-connected Zener diodes will not protect the lamps against normal failure caused by normal current flows, but-will protect against failures due to excessive current surges associated with the failure of associated lamps. No suggestion appears therein of even any recognition whatsoever that the problems confronting Applicant even existed let alone any suggestion of any mechanism or technique whatsoever which would provide a solution to the problems successfully achieved by applicant in a very simple and effective manner.
Various other attempts have heretofore been made to provide various types of shunts in parallel with the filament of each bulb, whereby the string will continue to be illuminated whenever a bulb has burned out, or otherwise provides an open circuit condition. However, to the knowledge of Applicant, none of such arrangements have been practical enough from a commercial standpoint to ever become available on the marketplace.
Typical of such arrangements are found in U.S. patents RE 34,717; 1,024,495; 2,072,337; 2,760,120; 3,639,805; 3,912,966; 4,450,382; 4,682,079; 4,727,449; 5,379,214; and 5,006,724, together with English patent 12,398; Swiss patent 427,021 and French patent 884,370.
Of the foregoing prior art patents, the Fleck 449, Harnden '966, and the Swiss '021 patents appear, at first blush, to probably be the most promising in the prior art in indicating defective bulbs in a string by the use of filament shunt circuits and/or devices of various types which range from polycrystalline materials, to powders, and to metal oxide varistors, and the like, which provide for continued current flow through the string, but at either a higher or a lower level. The reason for this is because of the fact that the voltage drop occurring across each prior art shunt is substantially a different value than the value of the voltage drop across the incandescent bulb during normal operation thereof.
Some of these prior art shunts cause a reduced current flow in the series string because of too high of a voltage drop occurring across the shunt when a bulb becomes inoperable, either due to an open filament, a faulty bulb, a faulty socket, or simply because the bulb is not mounted properly in the socket, or is entirely removed or falls from its respective socket. However, other shunt devices cause the opposite effect due to an undesired increase in current flow. For example, when the voltage dropped across a socket decreases, then a higher voltage is applied to all of the remaining bulbs in the string, which higher voltage results in higher current flow and a decreased life expectancy of the remaining bulbs in the string. Additionally, such higher voltage also results in increased light output from each of the remaining bulbs in the string, which may not be desirable in some instances. However, when the voltage dropped across a socket increases, then a lower voltage is applied to all of the remaining bulbs in the series connected string, which results in lesser current flow and a corresponding decrease in light output from each of the remaining bulbs in the string. Such undesirable effect occurs in most of the prior art attempts, including those which, at first blush, might be considered the most promising techniques, especially the proposed use of a diode in series with a bilateral switch in the Fleck '449 patent, or the proposed use of a metal oxide varistor in the above Harnden '966 patent, or the use of the proposed counter-connected is rectifiers in the Swiss '021 patent.
For example, in the arrangement suggested in the above Fleck '449 patent, ten halogen filled bulbs, each having a minimum 12-volt operating rating, are utilized in a series circuit. The existence of a halogen gas in the envelope, permits higher value current flow through the filament with the result that much brighter light is obtainable in a very small bulb size. Normally, when ten 12-volt halogen bulbs are connected in a series string, the whole string goes dark whenever a single bulb fails and does not indicate which bulb had failed. To remedy this undesirable effect, Fleck provided a bypass circuit across each halogen filled bulb which comprised a silicon bilateral voltage triggered switch in series with a diode which rectifies the alternating-current ("A.C.") supply voltage and thereby permits current to flow through the bilateral switch only half of the time, i.e., only during each half cycle of the A.C. supply voltage. It is stated in Fleck that when a single bulb burns out, the remaining bulbs will have "diminished" light output because the diode will almost halve the effective voltage due to its blocking flow in one direction and conduction flow only in the opposite direction. Such substantially diminished light output will quite obviously call attention to the failed bulb, as well as avoid the application of a greater voltage which would decrease the life of the remaining filaments. However, in actual practice, a drastic drop in brightness has been observed, i.e. a drop from approximately 314 lux illumination output to approximately 15 lux illumination output when one bulb "goes out". Additionally, it is stated by the patentee that the foregoing procedure of replacing a burned out bulb involves the interruption of the application of the voltage source in order to allow the switch to open and to resume normal operation after the bulb has been replaced. (See column 2, lines 19-22 therein.) Additionally, as such an arrangement does not permit more that one bulb to be out at the same time, certain additional desirable special effects such as "twinkling", and the like, obviously would not be possible.
In the arrangement suggested in Harnden '966 patent, Harnden proposes to utilize a polycrystalline metal oxide varistor as the shunting device, notwithstanding the fact that it is well known that metal oxide varistors are not designed to handle continuous current flow therethrough. Consequently, they are merely a so-called "one-shot" device for protective purposes, i.e. a transient voltage suppressor that is intended to absorb high frequency or rapid voltage spikes and thereby preventing such voltage spikes from doing damage to associated circuitry. They are designed for use as spike absorbers and are not designed to function as a voltage regulator or as a steady state current dissipation circuit. While metal oxide varistors may appear in some cases similar to back-to-back Zener diodes, they are not interchangeable and function very differently according to their particular use. In fact, the assignee of the Harnden '966 patent which was formerly General Electric Corporation and now is apparently Harris Semiconductor, Inc., states in their Application Note 9311: "They (i.e., metal oxide varistors) are exceptional at dissipating transient voltage spikes but they cannot dissipate continuous low level power." In fact, they further state that their metal oxide varistors cannot be used as a voltage regulator as their function is to be used as a nonlinear impedance device. The only similarity that one can draw from metal oxide varistors and back-to-back Zener diodes is that they are both bidirectional; after that, the similarity ends. It is further stated in Harnden that varistors preferably have a rating of 125% of that of the bulb rating and that such rating would result in a decreased "stress" across the remaining bulbs in the series string.
Properly interpreted, this so-called decreased stress results in a loss of illumination in the remaining bulbs. For example, in a 50 bulb string operating at 120 volts A.C., each bulb receives an average voltage of 2.4 volts RMS ("root mean square") or 3.39 peak volts. Since the varistor responds to the peak voltage, the varistor rating of 125% would be 4.24 volts, equivalent to 3.0 volts RMS. The difference between 2.4 volts and 3.0 volts for just a single bulb failure is quite significant, particularly when compounded by subsequent failures of other bulbs in, say, a 50 bulb series string which is strung on outdoor shrubbery, and the like, and is subjected to wind and other movements and, accordingly, is totally unsuitable for Applicant's intended purposes.
In the Swiss '021 patent, Dyre discloses a bilateral shunt device having a breakdown voltage rating that, when exceeded, lowers the resistance thereof to 1 ohm, or less. This low value of resistance results in a substantial increase in the voltage being applied to the remaining bulbs even when only a single bulb is inoperative for any of the reasons previously stated. Thus, when multiple bulbs are inoperative, a still greater voltage is applied to the remaining bulbs, thereby again substantially increasing their illumination, and consequently, substantially shortening their life expectancy.
Even though the teachings of the foregoing prior art have been available for many years to those skilled in the art, none of such teachings, either singly or collectively, have found their way to commercial application. In fact, miniature Christmas tree type lights now rely solely upon a specially designed bulb which is supposed to short out when becoming inoperative. Obviously, such a scheme is not always effective, particularly when a bulb is removed from its socket or becomes damaged in handling, etc. The extent of the extreme attempts made by others to absolutely keep the bulbs from falling from their sockets, includes the use of a locking groove formed on the inside circumference of the socket mating with a corresponding raised ridge formed on the base of the bulb base unit. While this particular locking technique apparently is very effective to keep bulbs from falling from their respective sockets, the replacement of defective bulbs by the average user is extremely difficult, if not sometimes impossible, without resorting to mechanical gripping devices which can actually destroy the bulb base unit or socket.
In Applicant's co-pending application Ser. No. 08/896,278 entitled SERIES CONNECTED LIGHT STRING WITH FILAMENT SHUNTING and filed on Jul. 7, 1997, which application is a continuation of application Ser. No. 08/653,979 filed May 28, 1996 which, in turn, is a continuation-in-part ("CIP") of application Ser. No. 08/560,472 filed Nov. 17, 1995 which, in turn, is a CIP of application Ser. No. 08/494,725 filed Jun. 26, 1995, all of which disclosures are incorporated herein, there is disclosed and claimed therein various novel embodiments which very effectively solve the prior art failures in various new and improved ways. For example, there is disclosed therein a series string of incandescent light bulbs, each having a silicon type voltage regulating shunting device connected thereacross which has a predetermined voltage switching value which is greater than the voltage normally applied to said bulbs, and which said shunt becomes fully conductive only when the peak voltage applied thereacross exceeds its said predetermined voltage switching value, which occurs whenever a bulb in the string either becomes inoperable for any reason whatsoever, even by being removed or falling from its respective socket, and which circuit arrangement provides for the continued flow of rated current through all of the remaining bulbs in the string, together with substantially unchanged illumination in light output from any of those remaining operative in the string even though a substantial number of total bulbs in the string are simultaneously inoperative for any combinations of the various reasons heretofore stated. There is disclosed therein various type of shunting devices performing the above desired end result, including back-to-back Zener, or so-called "avalanche" diodes, non-avalanche bilateral silicon switches, and conventional Zener diodes, one-half of which are electrically connected in one current flow direction and the remaining one-half being electrically connected in the opposite current flow direction.
However, the shunting components required by applicant to achieve the new and unexpected functional results are not yet readily available on the marketplace in sufficient quantities and from an insufficient number of quality suppliers to minimize the purchase price thereof to the extent there will likely be universal acceptance of applicant's light strings as a replacement to all existing light strings who utilize no shunting devices whatsoever.
For example, with a pair of back-to-back Zener diodes capable of being purchased in large quantities for as low as even 6.cent. each would result in an additional manufacturing cost of at least $3.00 for a typical 50-bulb light string. This translates to a retail price increase of more than 50% over that of a conventional 50-bulb string having no filament shunting which now sells at retail for less than $5.00 each. Thus, to insure universal or even widespread acceptance of applicant's novel light string, particularly from a typical household, it is desirable that the ultimate cost of the shunting device add no more than 2.cent. per each socket, which is highly unlikely with present day manufacturing technology applicable to quantity production of back-to-back Zener diodes, and probably would also require a commitment of substantial development costs in order to get the ultimate selling price that low, if in fact such low selling price is even achievable at all from a practical standpoint.